This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We are exploring aspects of photosynthetic efficiency in unicellular desert green algae and their closest known aquatic relatives. Desert green algae have evolved multiple independent times from aquatic ancestors, providing a statistically robust phylogenetic framework in which to test for suites of traits that correlate with evolutionary transitions of these eukaryotic microbes from water to land. Specifically, we are interested in photosynthetic efficiency. Light attenuation by terrestrial canopies tends to enrich the far-red wavelengths, which are preferentially absorbed by PSI, in lower canopy layers. In higher plants, the spatial segregation of PSII and PSI in granal-appressed and non-appressed thylakoid regions, respectively, minimizes the excitation transfer from PSII antennae to PSI (termed spillover), as well as the preferential use of excitation energy by PSI driven simply by its rapid reaction rates. Overall photosynthetic efficiency is enhanced by the separation of photosystems. More homogeneous distributions of PSI and PSII, and less-developed grana, in chloroplasts of many common aquatic green algae may improve photosynthetic function in water, where far-red wavelengths are the first wavelengths lost with depth. Attenuated light in deeper water is enriched with light preferentially absorbed by PSII, and the mixing of PSII and PSI in algal thylakoids may enhance excitation spillover from PSII to PSI, improving overall photosynthetic efficiency under water. The implicit idea is that grana stacking may correlate with green plant life on land, and a lack of stacking may correlate with green plant life in water. Closely related desert and aquatic green algae provide the opportunity to test this idea. We are using spinning disk confocal microscopy to scan multiple desert and aquatic taxa for grana stacks;the stacks show as punctate chlorophyll fluorescence because PSII (the main source of chlorophyll fluorescence we are detecting) is concentrated in grana stacks.